Synthesis and structural diversity of lanthanide amidate complexes and their catalytic activities for the ring-opening polymerization of rac-lactide

Anionic lanthanide amidate complexes, Li(THF)Ln[C6H5C(O)NC6H3(Pr-i)(2)](4)(THF)(n)center dot(sol)(m) (Ln = La (1), Nd (2), Sm (3), Yb (4), Y (5), n = 0. or 1; sol = THF or n-Hex), were synthesized for the first time by direct protonolysis reaction of LiLn((NPr2)-Pr-i)(4)(THF) with benzamide proligand, C6H5CONHC6H3(Pr-i)(2) (HL). It was confirmed by their characterization data that complexes 1-5 presented rich and varied structures with changeable coordination modes of the amidate ligands. On the other hand, similar reactions of HL with neutral homoleptic lanthanide amides, Ln[N(SiMe3)(2)](3), cleanly yielded the neutral lanthanide amidate complexes, {phC(O)NC6H3((i)pr)(2)}(2)Ln{OC(Ph)(C6H3((i)pr)(2))N-H-N(C6H3((i)pr)(2))C(Ph)O)(THF)(2) (Ln = La (6), Nd (7), Sm (8)). Single-crystal X-ray structural determination revealed that complexes 6-8 featured an unprecedented N-H-N bond between two of the amidate ligands, making it the most notable difference from normal neutral analogs. Complexes 1-8 were characterized by elemental analysis and IR spectroscopy, with additional H-1 NMR spectroscopy in the case of complexes 1, 5 and 6. Furthermore, both the anionic and neutral lanthanide amidate complexes proved to be efficient catalysts for the ring-opening polymerization of rac-lactide, affording heterotactic-rich polylactides. The anionic lanthanide amidate complexes exhibited significantly higher activities than their neutral counterparts due to the cooperative effect. (C) 2013 Elsevier B.V. All rights reserved.